Organic light emitting diodes are being increasingly widely used in general lighting, for example as large-area luminous surfaces (surface light source). A conventional organic light emitting diode 502 (OLED) (FIG. 5) includes an anode 514 and a cathode 518 with an organic functional layer system 516 therebetween. The organic functional layer system 516 may include one or a plurality of emitter layer(s) in which electromagnetic radiation is generated, one or a plurality of charge generating layer structure(s) each composed of two or more charge generating layers (CGL) for charge generation, and one or a plurality of electron blocking layers, also designated as hole transport layer(s) (HTL), and one or a plurality of hole blocking layers, also designated as electron transport layer(s) (ETL), in order to direct the current flow. The OLED emission can easily be varied via the operating current of the OLED. Adaptation to external and internal light conditions is possible as a result.
The external light conditions can change (short-term process) e.g. if the OLED in a room with a window is exposed to the diurnally variable sunlight. Furthermore, the emitted wavelength spectrum of the OLED is subject to ageing processes (long-term process), such that, depending on the OLED layer construction and processing, the luminance typically decreases with time. The decrease in the luminance is brought about e.g. by increased current densities or temperatures which occur during the operation of the OLED and can damage the organic system.
In order to keep constant the luminance in the environment of the OLED surface light source with time, the luminance in the OLED-illuminated room can be kept constant by manual dimming or external switched-on sensors with electronic circuit.
In one conventional method, readjustment of the emitted wavelength spectrum of the OLED is dispensed with.
In one conventional method, the wavelength spectrum is manually readjusted, i.e. an automatic readjustment is not possible without additional outlay. However, the manual readjustment allows only inaccurate coordination of the emission spectrum with light conditions actually present, as a result of which electrical energy is wasted unnecessarily and an incorrect lighting condition is used.
In one conventional method, external photodiodes, photoconductors, phototransistors, photothyristors or the like are used for detecting the entire radiation, with external interconnection/wiring. The requisite circuit complexity and expenditure in terms of additional costs are high, however.
In a further method (FIG. 5), at least two photosensors 504, 506 are integrated for simultaneous detection of internal and external brightnesses, and different orientations (back/front), in the case of a surface light source 502. One photosensor 504 is covered by a diaphragm 508 with regard to external light, in order to measure the internal brightness of the OLED. Another photosensor 506 measures the internal light and the external light, wherein the internal measurement is reduced by a coupling-out structure 512. The differentiation of internal light and external light is made more difficult by this method, since external light and also internal light are guided in the waveguiding substrate 510. Freedom of design is restricted as a result of the loss of luminous area and the arrangement of the diaphragm 508.
Also known are electrically switchable mirror layers: DE100312941A1, DE102007022090A1; and electrically switchable diaphragms/filters: J. Jacobsen et al., IBM System Journal 36 (1997) 457-463; B. Comiskey et al. Nature 394 (1998) 253-255; WO199803896A1; WO199841899A1; WO2010064165A1; WO2009053890A2 and EP1601030A2.